Magnetic recording media are widely used in audio tapes, video tapes, computer tapes, disks and the like. Magnetic media may use thin metal layers as the recording layers, or may comprise coatings containing magnetic particles as the recording layer. The latter type of recording media employs particulate materials such as ferromagnetic iron oxides, chromium oxides, ferromagnetic alloy powders and the like dispersed in binders and coated on a substrate. In general terms, magnetic recording media generally comprise a magnetic layer coated onto at least one side of a non-magnetic substrate (e.g., a film for magnetic recording tape applications).
In certain designs, the magnetic coating (or “front coating”) is formed as a single layer directly onto a non-magnetic substrate. In an alternative approach, the front coating is a dual-layer construction, including a support layer on the substrate and a thin magnetic layer (or “upper layer”) formed directly on the support or lower layer. With this construction, the lower layer is thicker than the magnetic layer. The support layer is typically non-magnetic and generally comprised of a non-magnetic powder dispersed in a binder. Conversely, the upper layer comprises a magnetic metal particle powder or pigment dispersed in a binder system. The formulation for the magnetic layer is optimized to maximize the performance of the magnetic recording medium in such areas as signal-to-noise ratios, pulsewidth, and the like.
Magnetic tapes may also have a backside coating applied to the opposing side of the non-magnetic substrate in order to improve the durability, electrical conductivity, and tracking characteristics of the media. As with the front coatings, the backside coatings are typically combined with a suitable solvent to create a homogeneous mixture which is then coated onto the substrate, after which the coating is dried, calendered if desired, and then cured. The formulation for the backside coating or layer also comprises pigments and a binder system.
The magnetic recording medium is formed on a non-magnetic substrate. Exemplary substrate materials useful for magnetic recording tapes include polyesters such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), a mixture of polyethylene terephthalate and polyethylene naphthalate; polyolefins (e.g., polypropylene); cellulose derivatives; polyamides; and polyimides. The substrate is generally tensilized in the downweb direction for linear recording systems and generally tensilized in the crossweb direction for helical systems. However, such tensilizations causes alignment of the polymeric chains in the direction of tensilization, which, in the case of a down web tensilized film, can adversely affect crossweb thermal expansion and hygroscopic expansion of the film and the magnetic recording medium formed on the film. This in turn has an adverse effect on the dimensional stability of the magnetic recording medium and the track density.
It would be desirable to have a biaxially tensilized substrate that would maximize the dimensional stability of the magnetic recording medium formed thereon. It would be beneficial for the final magnetic recording medium product to exhibit low hygroscopic expansion and low thermal expansion in order to improve the track density and minimize any overwriting of data. However, changes in the substrate can require changes in the coating thickness as well, which requires additional research and development.
It has now been discovered that a magnetic recording medium which includes a non-magnetic substrate having a front side and a backside, a longitudinal direction and a crossweb direction, with a particulate/binder magnetic layer formed over the front side of the substrate, wherein the magnetic medium has a cross web dimensional difference from the magnetic recording head used therewith of less than 900 microns/meter over a temperature range of about 35 degrees, and over a 70% relative humidity range will provide superior smoothness and recording performance.
It has further been discovered that a magnetic recording medium using a substrate that comprises a biaxially tensilized substrate such as a polyethylene naphthalate, rather than a machine direction semi-tensilized polyethylene naphthalate, will provide additional dimensional stability without requiring changes in the substrate or coating thicknesses when the substrate is tensilized such that the thermal expansion of the tape is approximately equal to the thermal expansion of the magnetic recording head.